The UK's coronavirus alert level is to be raised from 4 to 5 - the highest level - for the first time.
It comes as the chief medical officers of England, Scotland, Wales and Northern Ireland have warned the NHS could be overwhelmed within 21 days "in several areas" without further action.
Level 5 or "red" means there is a "material risk of healthcare services being overwhelmed", compared to Level 4 when transmission of the virus is "high or rising exponentially".
The COVID-19 alert level refers to the threat of the epidemic and is separate from the tiering system in England.
It is set by the Joint Biosecurity Centre, which includes all four nations' chief medical officers, and determines how strict coronavirus restrictions should be.
The chief medical officers, along with NHS England's national medical director Professor Stephen Powis, agreed to raise the alert level to 5.
Watch: Boris Johnson announces new national lockdown for England
In a joint statement, they said: "Many parts of the health systems in the four nations are already under immense pressure.
"There are currently very high rates of community transmission, with substantial numbers of COVID patients in hospitals and in intensive care.
"Cases are rising almost everywhere, in much of the country driven by the new more transmissible variant.
"We are not confident that the NHS can handle a further sustained rise in cases and without further action there is a material risk of the NHS in several areas being overwhelmed over the next 21 days."
However, despite the "immense pressure" on the NHS, the chief medical officers stressed that people can still receive life-saving treatment, adding: "It is absolutely critical that people still come forward for emergency care."
"If you require non-urgent medical attention, please contact your GP or call NHS111," they said.
The coronavirus alert system was created in May, when the government mapped a way out of the initial lockdown - moving the alert level from 4 to 3 at that time.
Monday's raising of the alert level comes as Prime Minister Boris Johnson announced a new national lockdown for England in light of the rapid spread of the new variant of the virus.
Scotland earlier announced it will lockdown almost all of the country from midnight on Tuesday, with schools closed until at least 1 February.
Northern Ireland's First Minister Arlene Foster was due to join a call with the UK government ahead of this evening's announcement.
The closure of schools would be "part of the discussions", Ms Foster said.
The Welsh government announced on Monday evening that all schools and colleges will move to online learning until 18 January.